With the increasing popularity of electronic devices, such as laptop computers, portable digital assistants, digital cameras, mobile phones, digital audio players, video game consoles and the like, nonvolatile memory usage has continued to increase. Nonvolatile memories come in various types, including flash memories. Flash memories are widely used for information storage in electronic devices such as those mentioned above and others.
In conventional flash memories, data is stored in an array of individual memory cells, each of which includes a charge storage structure, such as a charge trap or floating gate. Generally speaking, with floating gate cells, each of the memory cells has two gates. One gate, the control gate, is analogous to the gate in a MOSFET. The other gate, the floating gate, is insulated all around by an oxide layer and is coupled between the control gate and the substrate. Because the floating gate is insulated by an oxide layer, any electrons placed on it (e.g., by tunneling) get trapped there and thereby enable the storage of data. More specifically, when electrons are stored on the floating gate, their presence modifies, by partially canceling out, the electric field generated when a voltage is provided to the control gate. This results in the modification of the threshold voltage of the channel of the cell, since a higher voltage on the control gate is needed to enable an electrical current to flow between the source and the drain of the cell as compared with what would be needed if there were no electrons stored on the floating gate. If the number of electrons stored on the floating gate is sufficiently large, the resulting modified threshold voltage will inhibit electrical current from flowing between the source and the drain when the normal operating voltage is provided to the control gate. Hence, in a typical flash memory cell that stores a single bit of data, electrical current will either flow or not flow when a memory cell is being read by providing a voltage on the control gate, depending on the number of electrons on the floating gate. The flow or no flow of electrical current, in turn, translates to a stored bit of data having a value of 1 or 0, respectively. Some flash memories (or other non-volatile storage devices) include multi-level cells, where a single cell is configured to store multiple bits of data by programming the memory cell to one of more than two data states (e.g., where each of the data states corresponds to respective ranges of threshold voltages).
In the pursuit of greater storage capacity in ever smaller chips, flash memory density has been increasing over the years, in part due to the down scaling of the memory cell dimensions. The continued down scaling of electronic devices has created many challenges and opportunities, among them the quest for an ultra-thin gate oxide. One problem that sometimes results from a thin gate oxide is leakage current. For example, when the oxide layer surrounding the floating gate of a flash memory cell is very thin, electrons stored on the floating gate may leak out (e.g., from the floating gate to the control gate and the word line that is coupled to the control gate, and eventually to ground), thus changing the originally stored bit of data having a value of 0, for example, into a bit of data having a value of 1.
The continued down scaling of electronic devices also tends to decrease the physical separation of components on an integrated circuit chip. For example, in a memory device, the tight word line to word line pitch may increase the possibility of leakage current from one word line to another. This may particularly be true in flash-type memories where one word line is charged to a high voltage (e.g., 10V) while the neighboring word lines and other components remain at a lower voltage (e.g., 0V, 2.3V, 5V, etc.). The high voltage may be generated in a voltage source (e.g., a charge pump or a voltage shifter) circuit and may be provided to a word line in a flash memory array to, for example, program one or more memory cells coupled to the word line. The world line charged to a high voltage may therefore induce leakage current to adjacent word lines and/or to other nearby components of the flash memory device. Although some leakage current can be tolerated in some cases, leakage current above a certain threshold may improperly alter the operation of some devices. If, for example, a particular word line in a flash memory array has a leakage path to a neighboring word line, the leaked current may alter the data stored in the floating gates of the neighboring word line. As another example, excessive leakage current may increase the power consumed (and thus the heat generated) by an integrated circuit chip.
In order to identify unacceptable leakage current levels (including the types of leakage current mentioned briefly above, and others), some integrated circuit chips may be tested during manufacturing. In a flash memory device, for example, a leakage current test may be conducted to measure the leakage current on each of the word lines to determine whether the leakage current from each word line is above a certain threshold and thus unacceptable.